Hmm. Do you mean it's dropping as far as 2A, or 9A? This would normally imply heat issues (usually due to bad connections, sometimes due to stuff being in direct sunlight). Is the outlet or plug adapter hot while charging?
If the screen is saying bad extension cord or connection, it's
probably because it's detecting too large of a voltage drop? My own voltage varies a lot but I haven't seen that error before. This
could be voltage fluctuation by the utility company (like in my case), within your home (e.g. if you have large electrical loads elsewhere), or simply the circuit powering the car. The last two cases shouldn't normally happen and could indicate something is physically wrong with your electrical.
Check what
voltage it's reporting as well when charging (at both 12A and reduced amounts), but most importantly start taking steps to make sure your electrical in your house is safe.
---
The reason why the miles per hour changes so much is because it shows the
rounded net power delivered, which follows approximately:
Rated Miles per Hour = (Voltage x Amps x 0.94) - 300
And then divides it by a Wh/mi constant that the car is rated for (its efficiency) to represent it in miles per hour. Pulling a random one since I don't know what model you have, let's say that's 245Wh/mi
The 300W reduction is for overheads like running the coolant pumps, while the 0.94 is to reflect an approximately 94% efficient power conversion (which seems correct for my car over a range of charging setups).
So at 12A:
Rated MPH = 115*12*0.94 - 300 = about 997W or 4.07mph (would probably show as 4)
And at 9A:
Rated MPH = 117*9*0.94 - 300 = about 690W or 2.82mph (would probably show as 3)
I used a lower voltage for the higher current because voltage sag occurs. Note that while 9A is 25% less than 12A, the power going into the battery is actually about 31% less. This is mostly because at lower charging powers, that 300W overhead becomes a larger portion of the power going into the car.
Now, the same reason it varies by the
current, it can also change on voltage. As above, it might be sensing too large of a voltage drop, which would also mean you are receiving low or variable voltage, which is probably swinging that miles per hour number down as well.
---
I'll try not to get too opinion-y here (neither mine nor others is going to be overly helpful in the end!), but as far as what's
healthiest, just charge it every night assuming you use it most days and aren't charging over 90%. There is no health benefit to letting it drop to, say, 20%. But there
is a potential negative impact on health, however small it may be.
